ESTIMATION OF DRAG COEFFICIENT AT LOW WIND SPEEDS OVER THE MONSOON TROUGH LAND REGION DURING MONTBLEX-90

Using observational data acquired during the Monsoon Trough Boundary L ayer Experiment (MONTBLEX-90), the drag coefficient C-D is estimated o ver two land stations, Jodhpur (arid region) and Kharagpur (moist regi on), by eddy correlation of velocity data from sonic anemometers. At l ow winds C-D varies with wind speed according to a power law, which at Jodhpur takes the form C-D=0.04 (U) over bar(10)(-1.06) ((U) over bar (10) being the mean wind speed at 10 m). The large values of the drag coefficient at low winds are associated with both near-neutral and str ongly unstable situations. On the other hand, the C-D values at Kharag pur are less than at Jodhpur by nearly 50% for wind speeds > 3ms(-1), and cannot be explained by the different roughness lengths at the two sites.